1. Field of the Invention
The present invention relates to a switching amplifier, containing a signal converter which converts an analog input signal into at least one sequence of pulses, and two or more switching stages, each possessing a drive transformer, wherein the primary winding of each drive transformer is connected by means of two or more controllable switching elements to a supply-voltage source, and the secondary winding of the drive transformer is separated from the primary winding in a manner such that no conductive connection exists between them, as well as containing a control transformer for the switching elements, wherein the primary winding of the control transformer is connected to the signal converter and its secondary windings are connected to the control electrodes of respective switching elements, wherein the supply-voltage input terminals of the switching stages, on the primary side are parallel-connected to the same supply-voltage source and the signal-output terminals, on the secondary side, are series-connected by means of rectifier assemblies, and further containing a low-pass filter, which is connected to the series-connected circuit-assembly, from the free end of which filter the amplifier output is derived.
2. Description of the Prior Art
Switching amplifiers of the type described above exhibit the high efficiency of the Class D amplifiers, and due to the fact, in the switching stages, the secondary winding is separated from the primary winding in a manner such that no conductive connection exists between them, and due to the fact that the output terminals of the switching stages are series-connected, these switching amplifiers facilitate the generation of high-power output signals at very high voltages. For example, a switching amplifier is described in German Offenlegungsschrift No. 2,935,445, which is designed as a modulating amplifier for a radio-station transmitter with an output power in the region of 250 kw.
On account of the saturation of the transformer, the permissible pulse duty factor of each switching stage is, at most, 1:1, which corresponds to a power output of 50%. In the switching amplifier described, each two switching stages accordingly form an amplifier channel, in which the switching elements and transformers of the switching stages are alterately switched, or excited, this alternate switching/excitation enabling the amplifier channel to operate at a power output of 100%. For this purpose, the analog input signal is converted into one or more pairs of pulse-sequences, the two pulse-sequences of each pair being composed of duration-modulated pulses, and being phase-shifted by 180.degree., and each switching stage is controlled by one of these pulse-sequences. In switching amplifiers with a plurality of amplifier channels, it is possible to trigger mutually corresponding switching stages in parallel, or a plurality of oppositely phase-shifted pairs of pulse-sequences can be generated, and the switching amplifiers of each channel are controlled by a pair of pulse-sequences assigned thereto. It is self-explanatory that, in the operating mode described, the switching elements and the transformers of each switching stage are switched on, or excited, once during each cycle of the pulse-sequence, with switching losses arising in a manner which is unrelated to the length of time over which the switching elements and transformers are switched on or excited.
In order to reduce these switching losses, a switching amplifier has also already been proposed, in which the signal converter generates, at preset time intervals, a number of pairs of pulse-sequences which corresponds to the instantaneous value of the amplitude of the analog signal. The two pulse-sequences of each pair of pulse sequences are likewise phase-shifted through 180.degree., and are composed of pulses having a constant duration and a pulse duty factor corresponding to 1:1. As a result of using pulses having a maximum duration, and by "modulating" the number of the pairs of pulse-sequences, it is possible to minimize the number of occasions on which the transformers require to be switched, or excited, during each cycle of pulse-sequences, and to reduce the switching losses accordingly.
For the two switching amplifiers which have been described, it holds that the number of amplifier channels required is determined on the one hand, by the specified maximum output voltage and output power and, on the other hand, by the allowable supply voltage. In order to construct switching amplifiers with high output voltages and large power outputs, it is necessary to spend comparatively large amounts on hardware, this expenditure being virtually doubled by the need for each amplifier channel to contain two switching stages.